the effects of a 

 concavo-convex 

 mirror* 



238 ANSWERS TO OPTICAL QVESTIONS. 



Explanation of drawn perpendicularly acrofs the (hortef! arc of convexity thai 

 palTes through the vertex ; and thefe hnes to he mdefinitely 

 near each other; — or to fpeak lefs mathematically, if the mir- 

 ror were divided into fmall parallel ftripes, drawn ri^ht acrofs 

 its concavity ; — each of thofe fmall zones or ftripes will refledl 

 the light of any diftant obje^ to a focus before the mirror with- 

 out any fenfible error; whence it will diverge in a kind of flat 

 pencil; and thefe pencils themfelves will not be parallel to each 

 other, but will diverge from a point behind the mirror or vir- 

 tual focus; fo that this laft divergence of the pencils will be at 

 right angles to the divergence of the rays that compofe them. 

 If therefore we attend to the portion of light which enters the 

 eye from what may be taken to be a radiant point of the image, 

 in this cafe, we (hall perceive that they cannot be brought to a 

 correct focus on the retina. For if the eye be adjufted to the 

 place of convergence before the mirror, the focal fpot will be 

 elongated in the dire6lion of the arc of convexity; and on the 

 contrary, if the eye be adjufted to the virtual focus of the con- 

 vex, or point of divergence, the rays of the pencils will not 

 be duly collected, but will render the fpot on the retina oblong 

 in the diredion of the arc of concavity. Neverthelefs, if the 

 dillance of the eye be fo great, as that the difference of ad. 

 fuHment for thefe two points (hall not produce any fenfible ef- 

 feci of this kind, the whole image will appear fufficiently neat 

 and diftin6l, though a little deformed; that is to fay, thofe di« 

 menfions which are governed by the concavity, will be fome» 

 what greater than thofe governed by the convexity; becaufe 

 .as we may affirm, the concave mirror is nearer the eye than 

 the convex. From (his general explanation, it will be eafily 

 underftood that tho image raufl difnppear and become utterly 

 confufed by the quantity of crofs aberation, when any attempt 

 is made to examine it with a convex lens; and that it will be 

 impollible to form an image of the fun or of any other objed 

 upon the furface of paper or other fimilar material. In fa6l 

 the refleded light from the fun confi/dered as a himinous point, 

 will converge before the mirror to a line or cauftic curve of 

 which I have not yet confidered the properties ; but which, if 

 the concave mirror were cylindrical, would be a right line at 

 the dillance of half the radius from the vertex. 



.J-j^ is not neceffary after what has been faid to enter into any 

 fuller explanation of the reafon why the im#ige is inverted, 

 (that is to fay either right handed w upfide-down) with regard 



r to 



